1. Field of the Invention
This invention relates to the treatment of subterranean, hydrocarbon-bearing formations with an improved coated proppant/aqueous gel carrier fluid system. The improved system is particularly useful in the treatment of subterranean formations for purposes of fracturing, gravel pack completions and forming consolidations of particulate material therein.
2. Description of the Prior Art
In the completion and operation of oil wells, gas wells, water wells and similar boreholes, it frequently is desirable to alter the producing characteristics of the formation by treating the well. Many such treatments involve the use of particulate material. For example, in hydraulic fracturing, solid particles, commonly referred to as "proppants" are used to maintain the fracture in an open or propped condition. Also, in sand control techniques, proppants may be placed in the well to prevent the influx or incursion of formation sand or particles.
It is common in the treatment of such formations to coat or encapsulate proppants with natural or synthetic film-forming materials. The coating on the surface of the proppants controls dispersion of particulate fragments or particulate fines. The generation of particulate fines can result from closure pressures exerted by the formation on the proppant. Controlling the migration of particulate fines enhances formation conductivity by preventing free particulate fines from plugging interstitial flow passages therein. While proppants which are characterized as elastic in nature, those which disintegrate upon crushing, benefit most by coating. Coating or encapsulating a proppant generally improves the overall downhole stability of the proppant.
Proppants may be selected from both organic and inorganic materials. Common organic materials include for example, wood chips, nut shells, crushed coke and coal. Inorganic materials include for example crushed rock, sand, and spherical pellets of glass.
Natural or synthetic film-forming materials include natural rubber, elastomers such as butyl rubber, and polyurethane rubber, various starches, petroleum pitch, tar, and asphalt, organic semisolid silicon polymers such as dimethyl and methylphenyl silicones, polyhydrocarbons such as polyethylene, polyproplylene, polyisobutylene, cellulose and nitrocellulose lacquers, vinyl resins such as polyvinylacetate, phenolformaldehyde resins, urea formaldehyde resins, acrylic ester resins such as polymerized esters resins of methyl, ethyl and butyl esters of acrylic and alpha-methylacrylic acids, epoxy resins, melamine resins, drying oils, mineral and petroleum waxes. Methods for coating proppants with such film forming materials include batch mixing and on-the-fly mixing.
Traditionally, carrier fluids, which suspend the coated proppants therein, included salt water or hydrocarbon liquid, such as diesel oil and gelled crude oil. More recently, carrier fluids of the aqueous gelled variety are used. Aqueous gelled carrier fluids include an aqueous gelling agent and a water component.
Aqueous gelling agents include cellulose derivatives and glactomannan derivatives. The popularity of aqueous gelling agents is primarily due to the cost, controllable rheology (crosslinking), environmental compatibility and increased proppant loading capability. Examples of crosslinking agents include aluminum, titanium and boron. Along with the popularity of aqueous gelled carrier fluids, the industry generally recognizes resinous materials and derivatives thereof as the preferred film forming materials.
When an aqueous gel carrier fluid is used to suspend and transport the coated proppant, upon appropriate placement of the coated proppant in the formation the viscosity of the carrier fluid is reduced or "broken" with a viscosity reducing agent or "breaker". The presence of unbroken gel in the formation decreases the conductivity of the proppant bed which can result in decrease productivity of the formation.
Traditional aqueous gelling agent breakers include enzymes and oxidizing breakers. Examples of such oxidizing breakers include ammonium; sodium or potassium persulfate; sodium peroxide; sodium chlorite; sodium, lithium or calcium hypochlorite; chlorinated lime; potassium perphosphate; sodium perborate; magnesium monoperoxyphthalate hexahydrate; and several organic chlorine derivatives such as N,N'-dichlorodimethylhydantion and N-chlorocyanuric acid and/or salts thereof.
At formation temperatures of between about 75.degree. F.-120.degree. F. and a pH range of generally between about 4 to 9, enzyme breakers are suitable. Above formation temperatures of about 140.degree. F. enzyme breakers are inadequate and oxidizing breakers are required. Generally, depending upon the temperature of the gelled carrier fluid, between about 0.5 and 5.0 lbs of oxidizing breaker, such as a persulfate breaker, per 1000 gal of aqueous gel is sufficient to break the carrier fluid in a non-resin coated proppant/aqueous gelled carrier fluid system.
Though resin coated proppants exhibit some compatibility with aqueous gel carrier fluids, when the pH and temperature of the gelled carrier fluid preclude the use of enzyme breakers, it is not uncommon to employ exceedingly high concentrations of oxidizing breakers to reduce the viscosity of these carrier fluids. Generally the concentration of oxidizing breaker required in a resin-coated proppant/aqueous gel system can be as high as 4 to 40 times the amount required for non-coated proppant/aqueous gel system.
Solutions to the oxidizing breaker problem presented by the resin-coated proppant/aqueous gel system have been as straight forward as adding increased amounts of oxidizing breaker to as complex as encapsulating the oxidizing breaker. The first alternative can result in uncontrolled breaks or limited breaking. Uncontrolled breaking can result in a "sand out" of the proppant prior to optimal placement in the target formation. Limited gelled fluid breaking can reduce formation conductivity by leaving unbroken gel in the formation and the proppant bed. Breaker encapsulation, while somewhat more successful than the former method, in many instances also requires the addition of excessive quantities of oxidizing breaker. In addition, the process of encapsulation increases the cost of the breaker. Thus there exists the need for a resin-coated proppant/aqueous gelled carrier fluid system which can be predictably broken with oxidizing breakers and wherein the concentration of the oxidizing breaker is similar to the concentration used in non-coated proppant/aqueous gelled carrier fluid systems.
The inventor has observed that reducing the concentration of free amides (generally primary and secondary amides) and free phenols in the aqueous gel carrier fluid reduces the concentrations of oxidizing breaker required to sufficiently break the aqueous gelled carrier fluid. The inventor has further observed that the aqueous gelled carrier fluid leaches free amides and phenols from certain film forming materials. The inventor has discovered that by using a proppant coating of a resole-type phenolic resin material which does not require hexamethylenetetramine (HEXA) for polymerization, concentrations of free amides in the aqueous gel are reduced. Additionally, as resole-type phenolic resins utilize a phenol prepolymer, the concentrations of free phenol in the carrier fluid are reduced. Thus, by using a resole-type phenolic resin coated proppant, the gelled fluid can be broken without the addition of large concentrations of oxidizing breaker or encapsulated oxidizing breakers